The human breast and ovarian cancer susceptibility gene, BRCA1, has recently been cloned. BRCA1 encodes a protein of unknown function, although it may be a tumor suppressor gene. The mutant alleles are predicted to be null alleles or loss-of-function mutations. To study the role of BRCA1 in tumorigenesis, we have created a mouse with a null allele at the BRCA1 locus. This targeted disruption results in an embryonic lethal phenotype. Therefore, we created mice with conditional knock-outs of BRCA1 only in the breast. The developmental and tumor phenotype of these mice are currently being studied.